Exhaust purification system

ABSTRACT

An exhaust purification system includes: an NOx reduction type catalyst, which is provided in an exhaust system; a temperature acquisition unit, which acquires a catalyst temperature of the NOx reduction type catalyst; and a regeneration treatment unit, which executes a catalyst regeneration to recover an NOx purification capacity, wherein the regeneration treatment unit alternately executes a rich control, in which an exhaust air fuel ratio is set to a rich state to raise a temperature of the NOx reduction type catalyst to a predetermined target temperature, and a lean control, in which the exhaust air fuel ratio is set to a lean state to lower the temperature of the NOx reduction type catalyst, and sets an execution period of the lean control based on a deviation between the catalyst temperature acquired by the temperature acquisition unit during the previous rich control and the target temperature.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage entry of PCT Application No.PCT/JP2015/075876, filed on Sep. 11, 2015, which claims priority toJapanese Patent Application No. 2014-186759, filed Sep. 12, 2014, thecontents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to an exhaust purification system.

BACKGROUND ART

In the background art, an NOx occlusion reduction type catalyst is knownas a catalyst which reduces and purifies a nitrogen compound (NOx) in anexhaust gas discharged from an internal combustion engine. When theexhaust gas is under a lean atmosphere, the NOx occlusion reduction typecatalyst occludes the NOx contained in the exhaust gas. When the exhaustgas is under a rich atmosphere, the NOx occlusion reduction typecatalyst detoxifies the occluded NOx through reducing and purifying byhydrocarbon contained in the exhaust gas, and discharges the NOx.

In the NOx occlusion reduction type catalyst, a sulfur oxide containedin the exhaust gas (hereinafter, referred to as SOx) is also occluded.There is a problem that when the SOx occlusion amount increases, the NOxpurification capacity of the NOx occlusion reduction type catalyst isreduced. For this reason, in a case where an SOx occlusion amountreaches a predetermined amount, in order that the SOx is desorbed fromthe NOx occlusion reduction type catalyst to recover the NOx occlusionreduction type catalyst from S-poisoning, it is necessary to regularlyperform the so-called SOx purge in which an unburned fuel is supplied toan upstream-side oxidation catalyst by the post injection or the exhaustpipe injection to raise an exhaust temperature to an SOx desorptiontemperature (for example, see Patent Literature 1).

CITATION LIST Patent Literature

[Patent Literature 1]: Japanese Unexamined Patent ApplicationPublication No. 2009-047086

[Patent Literature 2]: Japanese Unexamined Patent ApplicationPublication No. 2007-315225

[Patent Literature 3]: Japanese Unexamined Patent ApplicationPublication No. 2009-115038

SUMMARY OF INVENTION Technical Problem

A method, in which a rich control to set an exhaust air fuel ratio to arich state so as to raise the exhaust temperature and a lean control toset an exhaust air fuel ratio to a lean state so as to lower the exhausttemperature are alternately executed, is known as a method in which acatalyst temperature at the time of the SOx purge is kept in apredetermined temperature range (for example, see Patent Literature 1).However, if respective execution periods of the rich control and thelean control are not optimally controlled, the exhaust temperature isexcessively raised during the rich control, which may cause heatdeterioration of the NOx occlusion reduction type catalyst. In addition,when the exhaust temperature is excessively lowered during the leancontrol, the catalyst temperature may be hardly stabilized to the SOxdesorption temperature.

The disclosed system is made to effectively suppress that a catalysttemperature at the time of an SOx purge is excessively raised orlowered.

The disclosed system is an exhaust purification system including an NOxreduction type catalyst, which is provided in an exhaust system of aninternal combustion engine and reduces and purifies NOx in an exhaustgas; a temperature acquisition unit, which acquires a catalysttemperature of the NOx reduction type catalyst; and a regenerationtreatment unit, which executes a catalyst regeneration to recover an NOxpurification capacity of the NOx reduction type catalyst, wherein theregeneration treatment unit alternately executes a rich control, inwhich an exhaust air fuel ratio is set to a rich state so as to raisethe NOx reduction type catalyst to a predetermined target temperature,and a lean control, in which the exhaust air fuel ratio is set to a leanstate so as to lower a temperature of the NOx reduction type catalyst,and sets an execution period of the lean control by a PID control, basedon a deviation between a catalyst temperature acquired by thetemperature acquisition unit during the previous rich control and thetarget temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an entire configuration diagram illustrating an exhaustpurification system according to this embodiment.

FIG. 2 is a timing chart for describing an SOx purge control accordingto this embodiment.

FIG. 3 is a block diagram illustrating a setting process of a MAF targetvalue at the time of an SOx purge lean control according to thisembodiment.

FIG. 4 is a block diagram illustrating a setting process of a targetinjection amount at the time of an SOx purge rich control according tothis embodiment.

FIG. 5 is a timing chart for describing a catalyst temperatureadjustment control in the SOx purge control according to thisembodiment.

FIG. 6 is a timing chart for describing an NOx purge control accordingto this embodiment.

FIG. 7 is a block diagram illustrating a setting process of a MAF targetvalue at the time of an NOx purge lean control according to thisembodiment.

FIG. 8 is a block diagram illustrating a setting process of a targetinjection amount at the time of an NOx purge rich control according tothis embodiment.

FIG. 9 is a block diagram illustrating a process of an injection amountlearning correction of an injector according to this embodiment.

FIG. 10 is a flow diagram for describing a calculation process of alearning correction coefficient according to this embodiment.

FIG. 11 is a block diagram illustrating a setting process of a MAFcorrection coefficient according to this embodiment.

DESCRIPTION OF EMBODIMENTS

Hereinafter, an exhaust purification system according to one embodimentof the present invention will be described based on accompanyingdrawings.

As illustrated in FIG. 1, an injector 11 which directly injects highpressure fuel accumulated in a common rail (not illustrated) into acylinder is provided in each of cylinders of a diesel engine(hereinafter, simply referred to as an engine) 10. The fuel injectionamount or the fuel injection timing of the injector 11 is controlled inresponse to an instruction signal input from an electronic controller(hereinafter, referred to as ECU) 50.

An intake manifold 10A of the engine 10 is connected with an intakepassage 12 which introduces fresh air therein, and an exhaust manifold10B is connected with an exhaust passage 13 which derives an exhaust gasoutside. An air cleaner 14, an intake air amount sensor (hereinafter,referred to as a MAF sensor) 40, a compressor 20A of a variable capacitysupercharger 20, an intercooler 15, an intake throttle valve 16, and thelike are provided in order from an intake upstream side in the intakepassage 12. A turbine 20B of the variable capacity supercharger 20, anexhaust post-treatment device 30, and the like are provided in orderfrom an exhaust upstream side in the exhaust passage 13. In FIG. 1, areference numeral 41 denotes an engine speed sensor, a reference numeral42 denotes an accelerator opening sensor, and a reference numeral 46denotes a boost pressure sensor.

An EGR device 21 includes an EGR passage 22 which connects the exhaustmanifold 10B and the intake manifold 10A, an EGR cooler 23 which coolsan EGR gas, and an EGR valve 24 which adjusts an EGR amount.

The exhaust post-treatment device 30 is configured such that anoxidation catalyst 31, an NOx occlusion reduction type catalyst 32, anda particulate filter (hereinafter, simply referred to as a filter) 33are disposed in order from the exhaust upstream side in a case 30A. Anexhaust pipe injection device 34 which injects an unburned fuel (mainly,HC) into the exhaust passage 13 in response to the instruction signalinput from an ECU 50 is provided in the exhaust passage 13 on theupstream side from the oxidation catalyst 31.

For example, the oxidation catalyst 31 is formed by carrying anoxidation catalyst component on a ceramic carrier surface such as ahoneycomb structure. When an unburned fuel is supplied by the postinjection of the exhaust pipe injection device 34 or the injector 11,the oxidation catalyst 31 oxidizes the unburned fuel to raise theexhaust temperature.

For example, the NOx occlusion reduction type catalyst 32 is formed bycarrying an alkali metal and the like on a ceramic carrier surface suchas a honeycomb structure. The NOx occlusion reduction type catalyst 32occludes NOx in the exhaust gas when an exhaust air fuel ratio is in alean state, and reduces and purifies the occluded NOx by a reducingagent (HC and the like) contained in the exhaust gas when the exhaustair fuel ratio is in a rich state.

For example, the filter 33 is formed such that a plurality of cellssectioned by porous partition walls are disposed in a flowing directionof the exhaust gas, and the upstream side and the downstream side of thecells are sealed alternately. In the filter 33, PM in the exhaust gas iscollected in a pore or a surface of the partition wall, and when theestimation amount of PM deposition reaches a predetermined amount, theso-called filter-forced regeneration is performed which combusts andremoves the PM. The filter-forced regeneration is performed in such amanner that the unburned fuel is supplied to the oxidation catalyst 31on the upstream side by an exhaust pipe injection or the post injection,and the temperature of the exhaust gas flowing in the filter 33 israised to a PM combusting temperature.

A first exhaust temperature sensor 43 is provided on the upstream sidefrom the oxidation catalyst 31, and detects the temperature of theexhaust gas flowing in the oxidation catalyst 31. A second exhausttemperature sensor 44 is provided between the oxidation catalyst 31 andthe NOx occlusion reduction type catalyst 32, and detects thetemperature of the exhaust gas flowing in the NOx occlusion reductiontype catalyst 32. An NOx/lambda sensor 45 is provided on the downstreamside from the filter 33, and detects an NOx value and a lambda value ofthe exhaust gas passing through the NOx occlusion reduction typecatalyst 32 (hereinafter, referred to as an excess-air-ratio).

The ECU 50 performs various controls on the engine 10 and the like, andincludes a well-known CPU or a ROM, a RAM, an input port, an outputport, and the like. In order to perform the various controls, the sensorvalues of the sensors 40 to 45 are input to the ECU 50. The ECU 50includes a filter-forced regeneration controller 51, an SOx desorptiontreatment unit 60, an NOx desorption treatment unit 70, a MAF follow-upcontroller 80, an injection amount learning correction unit 90, and aMAF correction coefficient calculation unit 95 as partial functionalelements. In description, such functional elements are included in theECU 50 which is an integral hardware. However, any part thereof may beprovided in a separate hardware.

<Filter-forced Regeneration Control>

The filter-forced regeneration controller 51 estimates the PM depositionamount of the filter 33 from the travel distance of the vehicle, or thedifferential pressure across the filter detected by a differentialpressure sensor (not illustrated), and turns on a forced regenerationflag F_(DPF) when the estimation amount of PM deposition exceeds apredetermined upper limit threshold (see time t₁ of FIG. 2). When theforced regeneration flag F_(DPF) is turned on, the instruction signalwhich executes the exhaust pipe injection is transmitted to the exhaustpipe injection device 34, or the instruction signal which executes thepost injection is transmitted to each of the injectors 11, so that theexhaust temperature is raised to the PM combusting temperature (forexample, about 550° C.). The forced regeneration flag F_(DPF) is turnedoff when the estimation amount of PM deposition is reduced to apredetermined lower limit threshold (determination threshold) indicatingcombusting and removing (see time t₂ of FIG. 2). For example, thedetermination threshold in which the forced regeneration flag F_(DPF) isturned off may be set based on the upper limit elapsed time or the upperlimit cumulative injection amount from the start (F_(DPF)=1) of thefilter-forced regeneration.

<SOx Purge Control>

The SOx desorption treatment unit 60 is an example of a regenerationtreatment unit of the present invention, and executes a control(hereinafter, referred to the control as an SOx purge control) whichrecovers the NOx occlusion reduction type catalyst 32 from SOx-poisoningby setting the exhaust gas to a rich state so as to raise the exhausttemperature to a sulfur desorption temperature (for example, about 600°C.).

FIG. 2 illustrates a timing flowchart of the SOx purge control of thisembodiment. As illustrated in FIG. 2, the SOx purge flag F_(SP) whichstarts the SOx purge control is turned on simultaneously when the forcedregeneration flag F_(DPF) is turned off (see time t₂ of FIG. 2).Accordingly, a transition to the SOx purge control can be efficientlyperformed from a state where the exhaust temperature is raised by theforced regeneration of the filter 33, and the fuel consumption amountcan be reduced effectively.

In this embodiment, the enrichment of the exhaust gas is made by usingthe SOx purge control, for example, in a such a manner that the SOxpurge lean control that lowers the excess-air-ratio by an air-systemcontrol from a steady operating state (for example, about 1.5) to afirst target excess-air-ratio (for example, about 1.3) on a lean sidefrom a value equivalent to a theoretical air-fuel ratio (about 1.0), andthe SOx purge rich control that lowers the excess-air-ratio by theinjection control from the first target excess-air-ratio to a secondtarget excess-air-ratio on a rich side (for example, about 0.9) are usedin combination. Hereinafter, a detail description will be given aboutthe SOx purge lean control and the SOx purge rich control.

<Air-system Control of SOx Purge Lean Control>

FIG. 3 is a block diagram illustrating a setting process of a MAF targetvalue MAF_(SPL Trgt) at the time of the SOx purge lean control. A firsttarget excess-air-ratio setting map 61 is a map based on an engine speedNe and an accelerator opening degree Q (fuel injection amount of theengine 10). An excess-air-ratio target value λ_(SPL Trgt) (first targetexcess-air-ratio) at the time of the SOx purge lean controlcorresponding to the engine speed Ne and the accelerator opening degreeQ is set based on an experiment and the like, in advance.

First, the excess-air-ratio target value λ_(SPL Trgt) at the time of theSOx purge lean control is read from the first target excess-air-ratiosetting map 61 by using the engine speed Ne and the accelerator openingdegree Q as input signals, and is input to a MAF target valuecalculation unit 62. In addition, in the MAF target value calculationunit 62, the MAF target value MAF_(SPL Trgt) at the time of the SOxpurge lean control is calculated based on the following Equation (1).MAF _(SPL Trgt)=λ_(SPL Trgt) ×Q _(fnl corrd) ×Ro _(Fuel) ×AFR _(sto)/Maf _(corr)  (1)

In Equation (1), Q_(fnl corrd) indicates a learning-corrected (to bedescribed later) fuel injection amount (excluding the post injection),Ro_(Fuel) indicates a fuel specific gravity, AFR_(sto) indicates atheoretical air-fuel ratio, and Maf_(corr) indicates a MAF correctioncoefficient (to be described later).

The MAF target value MAF_(SPL Trgt) calculated by the MAF target valuecalculation unit 62 is input to a ramp treatment unit 63 when the SOxpurge flag F_(SP) is turned on (see time t₂ of FIG. 2). The ramptreatment unit 63 reads a ramp coefficient from ramp coefficient maps63A and 63B by using the engine speed Ne and the accelerator openingdegree Q as input signals, and inputs a MAF target ramp valueMAF_(SPL Trgt Ramp), in which the ramp coefficient is added, to a valvecontroller 64.

The valve controller 64 executes a feedback control that throttles theintake throttle valve 16 to the shutting side and opens the EGR valve 24to the open side such that an actual MAF value MAF_(Act) input from theMAF sensor 40 becomes the MAF target ramp value MAF_(SPL Trgt Ramp).

In this manner, in this embodiment, the MAF target value MAF_(SPL Trgt)is set based on the excess-air-ratio target value λ_(SPL Trgt) read fromthe first target excess-air-ratio setting map 61 and the fuel injectionamount of the injector 11, and an air system operation isfeedback-controlled based on the MAF target value MAF_(SPL Trgt).Accordingly, without providing the lambda sensor on the upstream side ofthe NOx occlusion reduction type catalyst 32 or without using a sensorvalue of the lambda sensor even when the lambda sensor is provided onthe upstream side of the NOx occlusion reduction type catalyst 32, theexhaust gas can be effectively lowered to the desired excess-air-ratiorequired for the SOx purge lean control.

When the fuel injection amount Q_(fnl corrd) after the learningcorrection is used as the fuel injection amount of the injector 11, theMAF target value MAF_(SPL Trgt) can be set by a feed-forward control toeffectively exclude influence such as the aged deterioration, theproperty change, or the individual difference of the injector 11.

When the ramp coefficient set in response to the operating state of theengine 10 is added to the MAF target value MAF_(SPL Trgt), thedeterioration of the drivability and the like caused by the misfire orthe torque fluctuation of the engine 10 due to the rapid change of anintake air amount can be effectively suppressed.

<Fuel Injection Amount Setting of SOx Purge Rich Control>

FIG. 4 is a block diagram illustrating a setting process of the targetinjection amount Q_(SPR Trgt) (injection amount per unit of time) of theexhaust pipe injection or the post injection in the SOx purge richcontrol. A second target excess-air-ratio setting map 65 is a map basedon the engine speed Ne and the accelerator opening degree Q. Theexcess-air-ratio target value λ_(SPR Trgt) (second targetexcess-air-ratio) at the time of the SOx purge rich controlcorresponding to the engine speed Ne and the accelerator opening degreeQ is set based on an experiment and the like, in advance.

First, the excess-air-ratio target value λ_(SPR Trgt) at the time of theSOx purge rich control is read from the second target excess-air-ratiosetting map 65 by using the engine speed Ne and the accelerator openingdegree Q as input signals, and is input to an injection amount targetvalue calculating unit 66. In addition, in the injection amount targetvalue calculating unit 66, the target injection amount Q_(SPR Trgt) atthe time of the SOx purge rich control is calculated based on thefollowing Equation (2).Q _(SPR Trgt) =MAF _(SPL Trgt) ×Maf _(corr)/(λ_(SPR Target) ×Ro _(Fuel)×AFR _(sto))−Q _(fnl corrd)  (2)

In Equation (2), MAF_(SPL Trgt) is a MAF target value at the time of alean SOx purge, and is input from the above-described MAF target valuecalculation unit 62. Q_(fnlRaw corrd) indicates a learning-corrected (tobe described later) fuel injection amount (excluding the post injection)before a MAF follow-up control is applied thereto, Ro_(Fuel) indicates afuel specific gravity, and AFR_(sto) indicates a theoretical air-fuelratio, and Maf_(corr) indicates a MAF correction coefficient (to bedescribed later).

When the SOx purge rich flag F_(SPR) (to be described later) is turnedon, the target injection amount Q_(SPR Trgt) calculated by the injectionamount target value calculating unit 66 is transmitted as the injectioninstruction signal to the exhaust pipe injection device 34 or theinjector 11.

In this manner, in this embodiment, the target injection amountQ_(SPR Trgt) is set based on the excess-air-ratio target valueλ_(SPR Trgt) read from the second target excess-air-ratio setting map 65and the fuel injection amount of the injector 11. Accordingly, withoutproviding the lambda sensor on the upstream side of the NOx occlusionreduction type catalyst 32 or without using a sensor value of the lambdasensor even when the lambda sensor is provided on the upstream side ofthe NOx occlusion reduction type catalyst 32, the exhaust gas can beeffectively lowered to the desired excess-air-ratio required for the SOxpurge rich control.

When the fuel injection amount Q_(fnl corrd) after the learningcorrection is used as the fuel injection amount of the injector 11, thetarget injection amount Q_(SPR Trgt) can be set by the feed-forwardcontrol to effectively exclude influence such as the aged deterioration,the property change, or the like of the injector 11.

<Catalyst Temperature Adjustment Control of SOx Purge Control>

As illustrated in times t₂ to t₄ of FIG. 2, the temperature of theexhaust gas (hereinafter, referred to as a catalyst temperature) flowingin the NOx occlusion reduction type catalyst 32 during the SOx purgecontrol is controlled by alternately switching on and off (rich andlean) of the SOx purge rich flag F_(SPR) which executes the exhaust pipeinjection or the post injection. When the SOx purge rich flag F_(SPR) isturned on (F_(SPR)=1), the catalyst temperature is raised by the exhaustpipe injection or the post injection (hereinafter, referred to a timethereof as an injection time T_(F INJ)). On the other hand, when the SOxpurge rich flag F_(SPR) is turned off, the catalyst temperature islowered by the stop of the exhaust pipe injection or the post injection(hereinafter, referred to a time thereof as an interval T_(F INT)).

In this embodiment, the injection time T_(F INJ) is set by reading avalue corresponding to the engine speed Ne and the accelerator openingdegree Q from an injection time setting map (not illustrated) createdthrough an experiment and the like, in advance. In the injection timesetting map, the injection time required to reliably lower theexcess-air-ratio of the exhaust gas obtained by an experiment and thelike, in advance to the second target excess-air-ratio is set inresponse to the operating state of the engine 10.

When the SOx purge rich flag F_(SPR) in which the catalyst temperatureis the highest is switched from the On state to the Off state, theinterval T_(F INT) is set through a feedback control. Specifically, theinterval T_(F INT) is processed by a PID control configured by aproportional control that changes an input signal in proportion to thedeviation ΔT between a target catalyst temperature and an estimatedcatalyst temperature when the SOx purge rich flag F_(SPR) is turned off,an integral control that changes the input signal in proportion to atime integral value of the deviation ΔT, and a differential control thatchanges the input signal in proportion to a time differential value ofthe deviation ΔT. The target catalyst temperature is set to such adegree as to desorb SOx from the NOx occlusion reduction type catalyst32. The estimated catalyst temperature may be estimated, for example,based on an inlet temperature of the oxidation catalyst 31 detected bythe first exhaust temperature sensor 43, an exothermic reaction insidethe oxidation catalyst 31 and the NOx occlusion reduction type catalyst32, and the like.

As illustrated in time t₁ of FIG. 5, when the SOx purge flag F_(SP) isturned on by the termination of the filter-forced regeneration(F_(DPF)=0), the SOx purge rich flag F_(SPR) is also turned on, and theinterval T_(F INT) feedback-calculated at the time of the previous SOxpurge control is reset temporarily. That is, at first time just afterthe filter-forced regeneration, the exhaust pipe injection or the postinjection is executed in response to the injection time T_(F INJ 1) setin the injection time setting map (see time from t₁ to t₂ of FIG. 5). Inthis manner, the SOx purge control starts from the SOx purge richcontrol without performing the SOx purge lean control, and thus a prompttransition to the SOx purge control can be performed and the fuelconsumption amount can be reduced without lowering the exhausttemperature raised by the filter-forced regeneration.

Next, when the SOx purge rich flag F_(SPR) is turned off with the lapseof the injection time T_(F INJ 1), the SOx purge rich flag F_(SPR) isturned off until the interval T_(F INT 1) set by the PID control elapses(see times t₂ to t₃ of FIG. 5). In addition, when the SOx purge richflag F_(SPR) is turned on with the lapse of the interval T_(F INT 1),the exhaust pipe injection or the post injection according to theinjection time T_(F INJ 2) is executed again (see time from t₃ to t₄ ofFIG. 5). Thereafter, the on-and-off switching of the SOx purge rich flagF_(SPR) is repeatedly executed until the SOx purge flag F_(SP) is turnedoff (see time t_(n) of FIG. 5) by the termination determination of theSOx purge control (to be described later).

In this manner, in this embodiment, the injection time T_(F INJ) inwhich the catalyst temperature is raised and the excess-air-ratio islowered to the second target excess-air-ratio is set from the map basedon the operating state of the engine 10, and the interval T_(F INT) inwhich the catalyst temperature is lowered is treated by the PID control.Accordingly, the catalyst temperature in the SOx purge control iseffectively kept in the desired temperature range required for a purge,and the excess-air-ratio can be reliably lowered to a target excessratio.

<Termination Determination of SOx Purge Control>

When any condition of (1) a case where the injection amount of theexhaust pipe injection or the post injection is accumulated since theSOx purge flag F_(SP) is turned on and then the cumulative injectionamount reaches a predetermined upper limit threshold amount, (2) a casewhere the elapsed time timed from the start of the SOx purge controlreaches a predetermined upper limit threshold time, and (3) a case wherethe SOx adsorbing amount of the NOx occlusion reduction type catalyst 32calculated based on a predetermined model equation including anoperating state of the engine 10, a sensor value of the NOx/lambdasensor 45, or the like as input signals is reduced to a predeterminedthreshold indicating SOx removal success is satisfied, the SOx purgecontrol is terminated by turning off the SOx purge flag F_(SP) (see timet₄ of FIG. 2 and time t_(n) of FIG. 5).

In this manner, in this embodiment, the upper limit of the cumulativeinjection amount and the elapsed time is set in the terminationcondition of the SOx purge control, so that it can be effectivelysuppressed that the fuel consumption amount is excessive in a case wherethe SOx purge does not progress due to the lowering of the exhausttemperature and the like.

<NOx Purge Control>

The NOx desorption treatment unit 70 is an example of the regenerationtreatment unit of the present invention. The NOx desorption treatmentunit 70 executes a control that recovers the NOx occlusion capacity ofthe NOx occlusion reduction type catalyst 32 by detoxifying the NOx,which is occluded in the NOx occlusion reduction type catalyst 32 whenthe exhaust gas is under a rich atmosphere, by reducing and purifying,and then discharging the NOx (hereinafter, referred to the control as anNOx purge control).

The NOx purge flag F_(NP) which starts the NOx purge control is turnedon when an NOx discharging amount per unit of time is estimated from theoperating state of the engine 10 and then an estimated accumulated valueΣNOx calculated by accumulating the NOx discharging amounts exceeds thepredetermined threshold (see time t₁ of FIG. 6). Alternatively, the NOxpurge flag F_(NP) is turned on in a case where an NOx purification rateof the NOx occlusion reduction type catalyst 32 is calculated from theNOx discharging amount on the catalyst upstream side estimated from theoperating state of the engine 10 and then an NOx amount on the catalystdownstream side detected by the NOx/lambda sensor 45, and the NOxpurification rate is lower than the predetermined determinationthreshold.

In this embodiment, the enrichment of the exhaust gas is made by usingthe NOx purge control, for example, in such a manner that the NOx purgelean control that lowers the excess-air-ratio by an air-system controlfrom a steady operating state (for example, about 1.5) to a third targetexcess-air-ratio (for example, about 1.3) on a lean side from a valueequivalent to a theoretical air-fuel ratio (about 1.0), and the NOxpurge rich control that lowers the excess-air-ratio by the injectioncontrol from a fourth target excess-air-ratio to the second targetexcess-air-ratio on a rich side (for example, about 0.9) are used incombination. Hereinafter, the detail description will be given about theNOx purge lean control and the NOx purge rich control.

<MAF Target Value Setting of NOx Purge Lean Control>

FIG. 7 is a block diagram illustrating a setting process of the MAFtarget value MAF_(NPL Trgt) at the time of the NOx purge lean control. Athird target excess-air-ratio setting map 71 is a map based on theengine speed Ne and the accelerator opening degree Q. Theexcess-air-ratio target value λ_(NPL Trgt) (third targetexcess-air-ratio) at the time of the NOx purge lean controlcorresponding to the engine speed Ne and the accelerator opening degreeQ is set based on an experiment and the like, in advance.

First, the excess-air-ratio target value λ_(NPL Trgt) at the time of theNOx purge lean control is read from the third target excess-air-ratiosetting map 71 by using the engine speed Ne and the accelerator openingdegree Q as input signals, and is input to the MAF target valuecalculation unit 72. In addition, in the MAF target value calculationunit 72, the MAF target value MAF_(NPL Trgt) at time of the NOx purgelean control is calculated based on the following Equation (3).MAF _(NPL Trgt)=λ_(NPL Trgt) ×Q _(fnl corrd) ×Ro _(Fuel) ×AFR _(sto)/Maf _(corr)  (3)

In Equation (3), Q_(fnl corr) indicates a learning-corrected (to bedescribed later) fuel injection amount (excluding the post injection),Ro_(Fuel) indicates a fuel specific gravity, AFR_(sto) indicates atheoretical air-fuel ratio, and Maf_(corr) indicates a MAF correctioncoefficient (to be described later).

The MAF target value MAF_(NPL Trgt) calculated by the MAF target valuecalculation unit 72 is input to a ramp treatment unit 73 when the NOxpurge flag F_(SP) is turned on (see time t₁ of FIG. 6). The ramptreatment unit 73 reads a ramp coefficient from ramp coefficient maps73A and 73B by using the engine speed Ne and the accelerator openingdegree Q as input signals, and inputs a MAF target ramp valueMAF_(NPL Trgt Ramp), in which the ramp coefficient is added, to a valvecontroller 74.

The valve controller 74 executes a feedback control that throttles theintake throttle valve 16 to the shutting side and opens the EGR valve 24to the open side such that the actual MAF value MAF_(Act) input from theMAF sensor 40 becomes the MAF target ramp value MAF_(NPL Trgt Ramp).

In this manner, in this embodiment, the MAF target value MAF_(NPL Trgt)is set based on the excess-air-ratio target value λ_(NPL Trgt) read fromthe third target excess-air-ratio setting map 71 and the fuel injectionamount of the injector 11, and an air system operation isfeedback-controlled based on the MAF target value MAF_(NPL Trgt).Accordingly, without providing the lambda sensor on the upstream side ofthe NOx occlusion reduction type catalyst 32 or without using a sensorvalue of the lambda sensor even when the lambda sensor is provided onthe upstream side of the NOx occlusion reduction type catalyst 32, theexhaust gas can be effectively lowered to the desired excess-air-ratiorequired for the NOx purge lean control.

When the fuel injection amount Q_(fnl corrd) after the learningcorrection is used as the fuel injection amount of the injector 11, theMAF target value MAF_(NPL Trgt) can be set by a feed-forward control toeffectively exclude influence such as the aged deterioration, theproperty change, or the like of the injector 11.

When the ramp coefficient set in response to the operating state of theengine 10 is added to the MAF target value MAF_(NPL Trgt), thedeterioration of the drivability and the like caused by the misfire orthe torque fluctuation of the engine 10 due to the rapid change of theintake air amount can be effectively suppressed.

<Fuel Injection Amount Setting of NOx Purge Rich Control>

FIG. 8 is a block diagram illustrating a setting process of the targetinjection amount Q_(NPR Trgt) (injection amount per unit of time) of theexhaust pipe injection or the post injection in the NOx purge richcontrol. A fourth target excess-air-ratio setting map 75 is a map basedon the engine speed Ne and the accelerator opening degree Q. Theexcess-air-ratio target value λ_(NPR Trgt) (fourth targetexcess-air-ratio) at the time of the NOx purge rich controlcorresponding to the engine speed Ne and the accelerator opening degreeQ is set based on an experiment and the like, in advance.

First, the excess-air-ratio target value λ_(NPR Trgt) at the time of theNOx purge rich control is read from the fourth target excess-air-ratiosetting map 75 by using the engine speed Ne and the accelerator openingdegree Q as input signals, and is input to an injection amount targetvalue calculating unit 76. In addition, in the injection amount targetvalue calculating unit 76, the target injection amount Q_(NPR Trgt) atthe time of the NOx purge rich control is calculated based on thefollowing Equation (4).Q _(NPR Trgt) =MAF _(NPL Trgt) ×Maf _(corr)(λ_(NPR Target) ×Ro _(Fuel)×AFR _(sto))−Q _(fnl corrd)  (4)

In Equation (4), MAF_(NPL Trgt) is a MAF target value at the time of alean NOx purge, and is input from the above-described MAF target valuecalculation unit 72. Q_(fnlRaw corrd) indicates a learning-corrected (tobe described later) fuel injection amount (excluding the post injection)before a MAF follow-up control is applied thereto, Ro_(Fuel) indicates afuel specific gravity, and AFR_(sto) indicates a theoretical air-fuelratio, and Maf_(corr) indicates a MAF correction coefficient (to bedescribed later).

When the NOx purge flag F_(SP) is turned on, the target injection amountQ_(NPR Trgt) calculated by the injection amount target value calculatingunit 76 is transmitted as the injection instruction signal to theexhaust pipe injection device 34 or the injector 11 (time t₁ of FIG. 6).The transmission of the injection instruction signal is continued untilthe NOx purge flag F_(NP) is turned off (time t₂ of FIG. 6) by thetermination determination of the NOx purge control (to be describedlater).

In this manner, in this embodiment, the target injection amountQ_(NPR Trgt) is set based on the excess-air-ratio target valueλ_(NPR Trgt) read from the fourth target excess-air-ratio setting map 75and the fuel injection amount of the injector 11. Accordingly, withoutproviding the lambda sensor on the upstream side of the NOx occlusionreduction type catalyst 32 or without using a sensor value of the lambdasensor even when the lambda sensor is provided on the upstream side ofthe NOx occlusion reduction type catalyst 32, the exhaust gas can beeffectively lowered to the desired excess-air-ratio required for the NOxpurge rich control.

When the fuel injection amount Q_(fnl corrd) after the learningcorrection is used as the fuel injection amount of the injector 11, thetarget injection amount Q_(NPR Trgt) can be set by the feed-forwardcontrol to effectively exclude influence such as the aged deterioration,the property change, or the like of the injector 11.

<Air-system Control Prohibition of NOx Purge Control>

In an area where the operating state of the engine 10 is in a low load,the ECU 50 feedback-controls the opening degree of the intake throttlevalve 16 or the EGR valve 24 based on a sensor value of the MAF sensor40. On the other hand, in an area where the operating state of theengine 10 is in a high load, the ECU 50 feedback-controls asupercharging pressure by the variable capacity supercharger 20 based ona sensor value of the boost pressure sensor 46 (hereinafter, referred tothe area as a boosting pressure FB control area).

In such a boosting pressure FB control area, a phenomenon occurs inwhich the control of the intake throttle valve 16 or the EGR valve 24interferes with the control of the variable capacity supercharger 20.For this reason, there is a problem that the intake air amount cannot bekept to the MAF target value MAF_(NPL Trgt) even when the NOx purge leancontrol is executed in which air system is feedback-controlled based onthe MAF target value MAF_(NPL Trgt) set in the above-described Equation(3). As a result, even when the NOx purge rich control to execute thepost injection or the exhaust pipe injection starts, theexcess-air-ratio may not be lowered to the fourth targetexcess-air-ratio (excess-air-ratio target value λ_(NPR Trgt)) requiredfor the NOx purge.

In order to avoid such a phenomenon, in the boosting pressure FB controlarea, the NOx desorption treatment unit 70 of this embodiment prohibitsthe NOx purge lean control to adjust the opening degree of the intakethrottle valve 16 or the EGR valve 24, and lowers the excess-air-ratioto the fourth target excess-air-ratio (excess-air-ratio target valueλ_(NPR Trgt)) only through the exhaust pipe injection or the postinjection. Accordingly, even in the boosting pressure FB control area,the NOx purge can be performed reliably. In addition, in the case, theMAF target value set based on the operating state of the engine 10 maybe applied to the MAF target value MAF_(NPL Trgt) of the above-describedEquation (4).

<Termination Determination of NOx Purge Control>

When any condition of (1) a case where the injection amount of theexhaust pipe injection or the post injection is accumulated since theNOx purge flag F_(NP) is turned on and then the cumulative injectionamount reaches a predetermined upper limit threshold amount, (2) a casewhere the elapsed time timed from the start of the NOx purge controlreaches the predetermined upper limit threshold time, and (3) a casewhere the NOx occlusion amount of the NOx occlusion reduction typecatalyst 32 calculated based on a predetermined model equation includingan operating state of the engine 10, a sensor value of the NOx/lambdasensor 45, or the like as input signals is reduced to a predeterminedthreshold indicating NOx removal success is satisfied, the NOx purgecontrol is terminated by turning off the NOx purge flag F_(NP) (see timet₂ of FIG. 6).

In this manner, in this embodiment, the upper limit of the cumulativeinjection amount and the elapsed time is set in the terminationcondition of the NOx purge control so that it can be reliably suppressedthat the fuel consumption amount is excessive in a case where the NOxpurge does not succeed due to the lowering of the exhaust temperatureand the like.

<MAF Follow-up Control>

In (1) a period of switching from the lean state of a regular operationto the rich state through the SOx purge control or the NOx purgecontrol, and (2) a period of switching the rich state to the lean stateof the regular operation through the SOx purge control or the NOx purgecontrol, the MAF follow-up controller 80 executes a control to correctthe fuel injection timing and the fuel injection amount of the injector11 in response to a MAF change (hereinafter, referred to the control asa MAF follow-up control).

<Injection Amount Learning Correction>

As illustrated in FIG. 9, the injection amount learning correction unit90 includes a learning correction coefficient calculating unit 91 and aninjection amount correcting unit 92.

The learning correction coefficient calculating unit 91 calculates alearning correction coefficient F_(Corr) of the fuel injection amountbased on an error Δλ between an actual lambda value λ_(Act) detected bythe NOx/lambda sensor 45 at the time of a lean operation of the engine10 and an estimated lambda value λ_(Est). When the exhaust gas is in thelean state, the oxidation reaction of HC does not occur in the oxidationcatalyst 31, and thus it is considered that the actual lambda valueλ_(Act) in the exhaust gas which passes through the oxidation catalyst31 and is detected by the NOx/lambda sensor 45 on the downstream sidematches with the estimated lambda value λ_(Est) in the exhaust gasdischarged from the engine 10. For this reason, in a case where theerror Δλ occurs between the actual lambda value λ_(Act) and theestimated lambda value λ_(Est), the error can be assumed to result froma difference between an instructed injection amount and an actualinjection amount in the injector 11. Hereinafter, the calculationprocess of the learning correction coefficient performed by the learningcorrection coefficient calculating unit 91 using the error Δλ will bedescribed based on the flow of FIG. 10.

In Step S300, it is determined based on the engine speed Ne and theaccelerator opening degree Q whether the engine 10 is in a leanoperating state. If the engine 10 is in the lean operating state, theprocedure proceeds to Step S310 in order to start the calculation of thelearning correction coefficient.

In Step S310, a learning value F_(CorrAdpt) is calculated by multiplyingthe error Δλ, which is obtained by subtracting the actual lambda valueλ_(Act) detected by the NOx/lambda sensor 45 from the estimated lambdavalue λ_(Est), by a learning value gain K₁ and a correction sensitivitycoefficient K₂ (F_(CorrAdpt)=(λ_(Est)−λ_(Act))×K₁×K₂). The estimatedlambda value λ_(Est) is estimated and calculated from the operatingstate of the engine 10 based on the engine speed Ne or the acceleratoropening degree Q. The correction sensitivity coefficient K₂ is read froma correction sensitivity coefficient map 91A illustrated in FIG. 9 byusing the actual lambda value λ_(Act) detected by the NOx/lambda sensor45 as an input signal.

In Step S320, it is determined whether an absolute value |F_(CorrAdpt)|of the learning value F_(CorrAdpt) is in a range of a predeterminedcorrection limit value A. In a case where the absolute value|F_(CorrAdpt)| exceeds the correction limit value A, this controlreturns to stop the present learning.

In Step S330, it is determined whether a learning prohibition flagF_(Pro) is turned off. The learning prohibition flag F_(Pro)corresponds, for example, to the time of a transient operation of theengine 10, the time of the SOx purge control (F_(SP)=1), the time of theNOx purge control (F_(NP)=1), and the like. It is because in a statewhere such a condition is satisfied, the error Δλ becomes largeraccording to the change of the actual lambda value λ_(Act) so that thelearning is not executed exactly. As for whether the engine 10 is in atransient operating state, for example, based on the time change amountof the actual lambda value λ_(Act) detected by the NOx/lambda sensor 45,a case where the time change amount is larger than the predeterminedthreshold may be determined as the transient operating state.

In Step S340, a learning value map 91B (see FIG. 9) based on the enginespeed Ne and the accelerator opening degree Q is renewed to the learningvalue F_(CorrAdpt) calculated in Step S310. More specifically, aplurality of learning areas sectioned in response to the engine speed Neand the accelerator opening degree Q are set on the learning value map91B. Preferably, such learning areas are set such that the range thereofis narrower as the area is used more frequently, and the range thereofis wider as the area is used less frequently. Accordingly, in thefrequently used area, a learning accuracy can be improved, and in theless-frequently used area, non-learning can be effectively suppressed.

In Step S350, the learning correction coefficient F_(Corr) is calculatedby adding “1” to the learning value read from the learning value map 91Bby using the engine speed Ne and the accelerator opening degree Q asinput signals (F_(Corr)=1+F_(CorrAdpt)). The learning correctioncoefficient F_(Corr) is input to the injection amount correcting unit 92illustrated in FIG. 9.

The injection amount correcting unit 92 executes the correction of thefuel injection amount by multiplying respective basic injection amountsof a pilot injection Q_(Pilot), a pre-injection Q_(Pre), a maininjection Q_(Main), an after injection Q_(After), by a post injectionQ_(Post) by the learning correction coefficient F_(Corr).

In this manner, a variation such as the aged deterioration, the propertychange, or the individual difference of the injectors 11 can beeffectively excluded by correcting the fuel injection amount of theinjector 11 with the learning value according to the error Δλ betweenthe estimated lambda value λ_(Est) and the actual lambda value λ_(Act).

<MAF Correction Coefficient>

The MAF correction coefficient calculation unit 95 calculates a MAFcorrection coefficient Maf_(corr) used to set the MAF target valueMAF_(SPL Trgt) or the target injection amount Q_(SPR Trgt) at the timeof the SOx purge control and to set the MAF target value MAF_(NPL Trgt)or the target injection amount Q_(NPR Trgt) at the time of the NOx purgecontrol.

In this embodiment, the fuel injection amount of the injector 11 iscorrected based on the error Δλ between the actual lambda value λ_(Act)detected by the NOx/lambda sensor 45 and the estimated lambda valueλ_(Est). However, since the lambda is a ratio of air and fuel, a factorof the error Δλ is not necessarily limited to the effect of thedifference between the instructed injection amount and the actualinjection amount in the injector 11. That is, the error Δλ of the lambdamay be affected by an error of the MAF sensor 40 as well as that of theinjector 11.

FIG. 11 is a block diagram illustrating a setting process of the MAFcorrection coefficient Maf_(corr) performed by the MAF correctioncoefficient calculation unit 95. A correction coefficient setting map 96is a map based on the engine speed Ne and the accelerator opening degreeQ, and the MAF correction coefficient Maf_(corr) indicating the sensorproperty of the MAF sensor 40 corresponding to the engine speed Ne andthe accelerator opening degree Q is set based on an experiment and thelike, in advance.

The MAF correction coefficient calculation unit 95 reads the MAFcorrection coefficient Maf_(corr) from the correction coefficientsetting map 96 by using the engine speed Ne and the accelerator openingdegree Q as input signals, and transmits the MAF correction coefficientMaf_(corr) to the MAF target value calculation units 62 and 72 and theinjection amount target value calculating units 66 and 76. Accordingly,the sensor property of the MAF sensor 40 can be effectively reflected toset the MAF target value MAF_(SPL Trgt) or the target injection amountQ_(SPR Trgt) at the time of the SOx purge control and the MAF targetvalue MAF_(NPL Trgt) or the target injection amount Q_(NPR Trgt) at thetime of the NOx purge control.

<Others>

The present invention is not limited to the above-described embodiment,and the invention may be modified appropriately without departing fromthe spirit and scope of the invention.

The invention claimed is:
 1. An exhaust purification system comprising:an NOx reduction catalyst, which is provided in an exhaust system of aninternal combustion engine and reduces and purifies NOx in an exhaustgas; a temperature detection sensor, which is provided in the exhaustsystem of the internal combustion engine and detects a temperature ofthe exhaust gas; and a controller, which executes a catalystregeneration to recover an NOx purification capacity of the NOxreduction catalyst, wherein when the catalyst regeneration is executed,the controller: alternately executes a rich control, in which an exhaustair fuel ratio is set to a rich state to raise a temperature of the NOxreduction catalyst to a predetermined target temperature, and a leancontrol, in which the exhaust air fuel ratio is set to a lean state tolower the temperature of the NOx reduction catalyst, and sets anexecution period of the lean control by a PID control, based on adeviation between a catalyst temperature acquired, based on thetemperature of the exhaust gas detected by the temperature detectionsensor at a timing at which the previous rich control is switched to thelean control, and the target temperature, executes the rich control byusing a post injection or an exhaust pipe injection, and sets a postinjection amount or an exhaust pipe injection amount based on an intakeair amount of the internal combustion engine, a predetermined targetexcess-air-ratio, and a fuel injection amount of the internal combustionengine, and sets a value, which is obtained by subtracting the fuelinjection amount of the internal combustion engine from a value obtainedby dividing the intake air amount of the internal combustion engine by aproduct of the target excess-air-ratio, a fuel specific gravity, and atheoretical air-fuel ratio, as the post injection amount or the exhaustpipe injection amount.
 2. The exhaust purification system according toclaim 1, wherein when the catalyst regeneration is executed, thecontroller sets an execution period of the rich control, based on anoperating state of the internal combustion engine.
 3. The exhaustpurification system according to claim 1, further comprising: a lambdasensor provided in the exhaust system of the internal combustion engine,wherein the controller corrects the fuel injection amount of theinternal combustion engine based on a difference between an estimatedlambda value estimated from the operating state of the internalcombustion engine and an actual lambda value detected by the lambdasensor, and wherein, when the catalyst regeneration is executed, thecontroller uses the corrected fuel injection amount as the fuelinjection amount of the internal combustion engine.
 4. The exhaustpurification system according to claim 1, wherein the catalystregeneration starts with the rich control.
 5. The exhaust purificationsystem according to claim 1, wherein the controller executes the leancontrol by stopping the post injection or the exhaust pipe injection.